Multi-spectral optoacoustic tomography (MSOT) is a method capable of resolving chromophoric agents with molecular specificity through several millimeters to centimeters of tissue. The technique is based on the optoacoustic phenomenon, i.e., generation of acoustic waves due to thermoelastic expansion caused by absorption of ultra-short optical pulses. Optoacoustic interrogation of living tissues is recently drawing vast attention due to its ability of preserving scattering-free spatial resolution deep in highly scattering tissues while providing rich contrast characteristic of optical spectrum. Over the last decade, optoacoustics has been considered for tissue imaging, mainly for resolving vascular contrast and the corresponding physiological changes, in particular oxy- and deoxy-hemoglobin, superficial vascular anatomy, brain lesion and functional cerebral hemodynamic changes, blood volume and oxygen consumption changes and the associated dynamic and functional neuronal activities.
The main difficulty arising from 3D optoacoustic imaging is the long acquisition times associated with recording signals from multiple spatial projections. In tomographic applications, to attain the best quality quantified reconstructions, the optoacoustic responses have to be collected from as many locations as possible around the imaged object or region of interest.
The generated optoacoustic waves need an acoustically-matched medium to effectively propagate and reach the detector's surface. As an example, arrangement of the object to be imaged and a detector element in a container with water have been proposed (see D. Razansky and V. Ntziachristos, “Med. Phys.,” Vol. 34, 2007, pp. 4293-4301). This technique may have the following disadvantages.
Placement of the detector element at different locations for collecting optoacoustic responses requires a translation stage, which is arranged in the container. Operating a translation stage in water may result in malfunctions and non-reproducible positioning. Furthermore, the container technique has been demonstrated with synthetic phantoms. In case of biological imaging, the container technique would require placement of the imaged object or region of interest into the water or other liquid media for coupling. This however imposes severe practical limitations. If, for instance, a living animal being imaged resides in water, variety of hygienic, epidemiological and general handling issues readily arise. Moreover, many areas of the body become inaccessible for imaging unless the animal is submerged entirely into the liquid, which requires specialized breathing accessories and further complicates the overall imaging configuration and abilities for fast and reliable data acquisition.
The generated optoacoustic responses are generally weak and the SNR is low, problems that are usually solved by multiple signal averaging, which only further complicates matters and makes imaging challenging for most applications, especially those dealing with living subjects. For instance, one could obtain a rough estimate on the order of magnitude of a typical optoacoustic disturbance by considering a simplified one-dimensional case of a short pulsed beam impinging upon absorbing half space. Under heat and temporal stress confinement conditions, pressure-rise distribution P(r) can be expressed as
            P      ⁡              (        r        )              =                                        β            ⁢                                                  ⁢                          C              s              2                                            C            P                          ⁢                              μ            a                    ⁡                      (            r            )                          ⁢                  ϕ          ⁡                      (            r            )                              =                                    Γμ            a                    ⁡                      (            r            )                          ⁢                  ϕ          ⁡                      (            r            )                                ,where the typical parameters for biological tissue are β=3·10−4 (° C.−1) for the thermal expansion coefficient; Cs=15·10−4 (cm s−1) for the speed of sound; Cp=4.186 (J g−1° C.−1) for the specific heat capacity at constant pressure; and μa=0.3 cm−1 for the optical absorption coefficient (see K. V. Larin et al. in “Journal of Physics D-Applied Physics,” vol. 38(15), 2005, p. 2645-2653). In a typical imaging scenario, illumination of tissue with maximal permissible fluence of ϕ=20 mJ/cm2 will only result in optoacoustic disturbances with less than 1 kPa magnitude on the tissue surface light is incident upon, which will translate into an order of magnitude lower detectable pressure variations on the detector's surface. If deep tissue imaging is of interest, the pressure variations will be further affected by light attenuation and acoustic dispersions, which will bring the signals down by another order of magnitude and more so that only few Pascals are available for detection. Finally, when considering multispectral optoacoustic tomography (MSOT) data acquisition, in which tomographic data is recorded at several different wavelengths, 3D image acquisition times readily become unrealistic.
Alternatively, spreading multiple small-area (point) detectors around the object along with signal averaging may only moderately improve the SNR and acquisition times. This is due to the fact that the noise floor is inversely proportional to square root of the number of signal averages. From the practical imaging perspective and SNR considerations it is therefore always desirable to increase detection sensitivity rather than reduce the thermal noise by using multiple measurements/averages.
It could therefore be helpful is to provide improved imaging devices for optoacoustic imaging of an object avoiding disadvantages and limitations of conventional techniques. In particular, it could be helpful for the improved imaging device to be capable of high performing and quantitative optoacoustic imaging. Furthermore, it could be helpful to provide improved imaging methods for optoacoustic imaging of an object. Of particular interest is the development of fast-acquisition imaging approaches for whole body imaging of model organisms and disease biomarkers in vivo that will enable studying fast-varying biological phenomena, real-time visualization of bio-marker distribution, pharmakokinetics, treatment responses, etc.